Saturday, June 27, 2009

8. Nuclear Physics

8. Nuclear Physics

1.What are Isotopes?Give examples.Isotopes are atoms of the same element having the same atomicnumber Z but different mass number A. The nuclei 1H1, 1H2 and 1H3are the isotopes of hydrogen. In other words isotopes of an elementcontain the same number of protons but different number of neutrons.2.What are Isobars?Give examples.Isobars are atoms of different elements having the same massnumber A, but different atomic number Z. The nuclei 8O16 and 7N16represent two isobars.3.What are Isotones?Give examples.Isotones are atoms of different elements having the same numberof neutrons. 6C14 and 8O16 are some examples of isotones.4.Define : “One atomic mass unit”One atomic mass unit is considered as one twelfth of the mass of carbon atom 6C12. Carbon of atomic number 6 and mass number 12 has mass equal to 12 amu. 1 amu = 1.66 × 10−27 kg.5.Calculate energy equivalence of one amu in electron-volt.Einstein’s mass energy relation is, E = mc2Here, m = 1 amu = 1.66 × 10−27 kg,c = 3 × 108 ms−1E = 1.66 × 10−27 × (3 × 108)2 JOne electron-volt (eV) is defined as the energy of an electron whenit is accelerated through a potential difference of 1 volt.1 eV = 1.6 × 10−19 coulomb × 1 volt 1 eV = 1.6 × 10−19 jouleeV = 931 × 106 eV = 931 million electronvolt = 931 MeVThus, energy equivalent of 1 amu = 931 MeV.6.Define mass defect.The difference in the total mass of the nucleons and theactual mass of the nucleus is known as the mass defect.mass of a nucleus, m < (Zmp + Nmn) Zmp + NmN – m = Δm, where Δm is the mass defect 7. Define : Binding EnergyWhen the protons and neutrons combine to form a nucleus, the mass that disappears (mass defect, Δm) is converted into an equivalent amount of energy (Δmc2). This energy is called the binding energy of the nucleus. Binding energy = [ZmP + Nmn – m] c2 = Δm c2 The binding energy of a nucleus determines its stability against disintegration. 8.Define Nuclear force There is some force in the nucleus which overcomes the electrostatic repulsion between positively charged protons and binds the protons and neutrons inside the nucleus. This force is called nuclear force.9.Define :Radioactivity (or) natural radioactivity.The phenomenon of spontaneous emission of highly penetrating radiations such as α, β and γ rays by heavy elements having atomic number greater than 82 is called radioactivity and the substances which emit these radiations are called radioactive elements.10.State four properties of α–rays (i) An α - particle is a helium nucleus consisting of two protons and two neutrons. It carries two units of positive charge. (ii) They move along straight lines with high velocities. (iii) They are deflected by electric and magnetic fields. (v) They affect photographic plates. (vi) They are scattered by heavy elements like gold. (vii) They produce fluorescence when they fall on substances like zinc sulphide or barium platinocyanide.11.State four Properties of β – rays (i) β–particles carry one unit of negative charge and mass equal to that of electron.Therefore, they are nothing but electrons. (ii) The β–particles emitted from a source have velocities over the range of 0.3 c to 0.99 c, where c is the velocity of light. (iii) They are deflected by electric and magnetic fields. (iv) The ionisation power is comparatively low (v) They affect photographic plates. (vi) They penetrate through thin metal foils and their penetrating power is greater than that of α−rays (vii) They produce fluorescence when they fall on substances like barium platinocyanide.12.State four Properties of γ – rays.(i) They are electromagnetic waves of very short wavelength. (ii) They are not deflected by electric and magnetic fields. (iii) They travel with the velocity of light. (iv) They produce very less ionisation. (v) They affect photographic plates. (vi) They have a very high penetrating power, greater than that of β-rays.(vii) They produce fluorescence. (viii) They are diffracted by crystals in the same way like X−rays are diffracted. 13.State Radioactive displacement law for α-decay.When a radioactive nucleus disintegrates by emitting an α-particle, the atomic number decreases by two and mass number decreases by four. The α-decay can be expressed as zXA à z−2YA−4 + 2He4 93 Example : Radium (88Ra226) is converted to radon (86Rn222) due to α−decay 88Ra226 à 86Rn222 + 2He414.State Radioactive displacement law for β−decayWhen a radioactive nucleus disintegrates by emitting a β− particle, the atomic number increases by one and the mass number remains the same. β−decay can be expressed as zXA à Z+1YA + −1e0 Example : Thorium (90Th234) is converted to protoactinium (91Pa234) due to β−decay => 90Th234 à 91Pa234 + −1e015.State Radioactive displacement law for γ−decayWhen a radioactive nucleus emits γ−rays, only the energy levelof the nucleus changes and the atomic number and mass numberremain the same. During α or β− decay, the daughter nucleus is mostly inthe excited state. It comes to ground state with the emission of γ−rays.Example : During the radioactive disintegration of radium (88Ra226)into radon (86Rn222), gamma ray of energy 0.187 MeV is emitted, whenradon returns from the excited state to the ground state .16.State Radioactive law of disintegration (or) Rutherford and Soddy lawRutherford and Soddy found that the rate of disintegration isindependent of physical and chemical conditions. The rate ofdisintegration at any instant is directly proportional to the number ofatoms of the element present at that instant. This is known asradioactive law of disintegration.N = N0 e−λt17.Define “half life period” of a radioactive element.The half life period of a radioactive element is defined as the timetaken for one half of the radioactive element to undergo disintegration.T½= 0.6931/λ18.Define “mean life period” of a radioactive element.The mean life of a radioactive substance is defined as the ratioof total life time of all the radioactive atoms to the total number ofatoms in it.Mean life =Sum of life time of all the atoms/Total number of atomsτ =1/λ λ = 0.6931 τ

19.Define “curie”The activity of a radioactive substance is generally expressed incurie. Curie is defined as the quantity of a radioactive substance whichgives 3.7 × 1010 disintegrations per second or 3.7 × 1010 becquerel. Thisis equal to the activity of one gram of radium.20.Classify neutrons based on kinetic energes.Neutrons are classified according to their kinetic energy as(a) slow neutrons and (b) fast neutrons. Both are capable of penetratinga nucleus causing artificial transmutation of the nucleus.Neutrons with energies from 0 to 1000 eV are called slow neutrons. The neutrons with an average energy of about 0.025 eV in thermal equilibrium are called thermal neutrons. Neutrons with energies in the range between 0.5 MeV and 10 MeV are called fast neutrons.21.Define : Artificial radioactivityThe phenomenon by which even light elements are maderadioactive by artificial or induced methods is called artificialradioactivity.5B10 + 2He4 à 7N13* + 0n17N13* à 6C13 + 1e013Al27 + 2He4 à 15P30* + 0n1

22.State the methods of production of artificial radio-isotopes(i) Artificial radio-isotopes are produced by placing the targetelement in the nuclear reactor, where plenty of neutrons are available.(1) 15P31 + 0n1 à 15P32* + γ, and (2) 11Na23 + 0n1 à 11Na24* + γ(ii) Another method of production of artificial radio-isotope is tobombard the target element with particles from particle acceleratorslike cyclotron.11Na23 + 1H2 à 11Na24* + 1H1

23.State the applications of radio-isotopes in medical field.Radio cobalt (Co60) --treatment ofcancer. Radio-sodium (Na24)-- detect the presence of blocks in blood vessels, to check the effective functioning of heart in pumping blood and maintaining circulation.Radio-iodine (I131) ---detection of thyroid gland and also for treatment& also used to locate brain tumours. Radio-iron (Fe59) --- diagnose anaemia.. Radio-phosphorous (P32) ---treatment of skin diseases.24.State the applications of radio-isotopes in Agriculture field.In agriculture, radio-isotopes help to increase the crop yields.Radio-phosphorous (P32) incorporated with phosphate fertilizer is addedto the soil. The plant and soil are tested from time to time. Phosphorous is taken by the plant for its growth and radio-phosphorousis found to increase the yield.Sprouting and spoilage of onions, potatoes, grams etc. areprevented by exposure to a very small amount of radiation. Certainperishable cereals remain fresh beyond their normal life span whenexposed to radiation.25.State the applications of radio-isotopes in Industrial field and Molecular biologyIn Industry, the lubricating oil containing radio-isotopes is usedto study the wear and tear of the machinery.In molecular biology radio-isotopes are used in sterilisingpharmaceutical and surgical instruments.26.What is Radio-carbon dating ?Living things take C14 which is radioactive, from food and air. However with death,the intake of C14 stops, and begins to decay. Hence the amount of C14 in the sample will enable the calculation of time of death i.e, the age of the specimen could be estimated. This is calledradio-carbon dating, employed in the dating of wooden implements, leather clothes, charcoal used in oil paintings, mummies and so on.

27.What are the biological effects of nuclear radition?The biological effects of nuclear radiation can be divided into threeGroups (i) Short term recoverable effects (ii) long term irrecoverable effects and (iii) genetic effect28.What are the factors that a human organism affected by radiation depends?The extent to which the human organism is damaged dependsupon (i) the dose and the rate at which the radiation is given and(ii) the part of the body exposed to it.29.What are the precautions to be taken while working radiation labs?The following precautions are to be taken for those, who areworking in radiation laboratories.(1) Radioactive materials are kept in thick−walled leadcontainer. (2) Lead aprons and lead gloves are used while working inhazardous area. (3) All radioactive samples are handled by a remote control process. (4) A small micro−film badge is always worn by the person and it is checked periodically for the safety limit of radiation.30.Define: “Roentgen”:The radiation exposure is measured by the unit called roentgen (R). Oneroentgen is defined as the quantity of radiation which produces 1.6 × 1012 pairs of ions in 1 gram of air.31.What is meant by artificial transmutation?Artificial transmutation is the conversion of one element intoanother by artificial methods.When nitrogen was bombarded with α-particles of sufficient energy, a rare isotope of oxygen (8O17) and a proton were formed.7N14 + 2He4 à 8O17 + 1H132.What are Particle accelerators?A particle accelerator is a device used to accelerate the chargedparticles, which are required in the study of artificial transmutation ofelements.(i) The first type belongs to electrostatic accelerators :The Cockcroft – Walton and Van de Graaff generators(ii) The second type is the cyclic or synchronous accelerator: Linear accelerator, cyclotron, betatron,synchrocyclotron and synchrotron.33.Define :Nuclear fissionThe process of breaking up of the nucleus of a heavier atom intotwo fragments with the release of large amount of energy is callednuclear fission.92U235 + 0n1 à 56Ba141 + 36Kr92 + 3 0n1 + Q34.Define: Chain reactionA chain reaction is a self propagating process in which the number of neutrons goes on multiplying rapidly almost in a geometrical progression.35..Define: Critical sizeCritical size of a system containing a fissile material is defined as the minimum size in which atleast one neutron is available for further fission reaction. The mass of the fissile material at the critical size is called critical mass. The chain reaction is not possible if the size is less than the critical size.36.What is nuclear reactor?A nuclear reactor is a device in which the nuclear fissionreaction takes place in a self sustained and controlled manner. Thefirst nuclear reactor was built in 1942 at Chicago USA.· Research reactors -supply neutrons for research purpose and for production of radio-isotopes.· Production reactors - convert fertile (non-fissile but abundant)material into fissile material.· The power reactor -converts nuclear fission energy into electric power37.Write note on :Fissile material or fuelThe fissile material or nuclear fuel generally used is 92U235. Butthis exists only in a small amount (0.7%) in natural uranium. Naturaluranium is enriched with more number of 92U235 (2 – 4%) and this lowenriched uranium is used as fuel in some reactors.pressurised heavy water reactors (PHWR) -natural uranium oxidepressurised light water reactors (PWR)-low enriched uraniumfast breeder test reactor (FBTR)-mixture of the carbides of uranium and plutoniumprototype fast breeder reactor (PFBR) -mixture of oxides of plutonium and uranium.Kamini - alloy of uranium and aluminium38.Write note on :ModeratorThe function of a moderator is to slow down fast neutronsproduced in the fission process having an average energy of about2 MeV to thermal neutrons with an average energy of about 0.025 eV,which are in thermal equilibrium with the moderator. Ordinary waterand heavy water are the commonly used moderators. A goodmoderator slows down neutrons by elastic collisions and it does notremove them by absorption.

39.Write note on :Control rodsThe control rods are used to control the chain reaction. They arevery good absorbers of neutrons. The commonly used control rods aremade up of elements like boron or cadmium.In our country, all the powerreactors use boron carbide (B4C), a ceramic material as control rod.40.Write note on :The cooling system.The cooling system removes the heat generated in the reactorcore. Ordinary water, heavy water and liquid sodium are the commonlyused coolants. A good coolant must possess large specific heat capacityand high boiling point.41.Why liquid sodium used as coolant in fast breeder reactors?In fast breeder reactors, liquid sodium is used as the coolant. Ahigh temperature is produced in the reactor core of the fast breederreactors. Being a metal substance, liquid sodium is a very goodconductor of heat and it remains in the liquid state for a very hightemperature as its boiling point is about 1000o C.

42.What are Breeder reactors?The process of producing more fissile material in a reactor thanconsumed during the operation of the reactor is called breeding.Such reactors are called breeder reactors.43.State the Uses of reactors.(1) Nuclear reactors are mostly aimed at power production,because of the large amount of energy evolved with fission.(2) Nuclear reactors are useful to produce radio-isotopes.(3) Nuclear reactor acts as a source of neutrons, hence used inthe scientific research.44.Define :Nuclear fusion.Nuclear fusion is a process in which two or more lighter nucleicombine to form a heavier nucleus.Example: 1H3 + 1H2 --> 2He4 + 0n1 + energy45.What are thermo nuclear reactions?The fusion process can be carried out only at a extremely hightemperature of the order of 107 K because, only at these very high temperatures the nuclei are able to overcome their mutual repulsion. Therefore before fusion, the lighter nuclei must have their temperature raised by several million degrees. The nuclear fusion reactions are known as thermo-nuclear reactions.46.What are Cosmic Rays?Ionising radiation coming from the outer space is the reason for leakage of charges. The ionizing radiation many times stronger than γ-rays entering the earth from all the directions from cosmic or interstellar space is known as cosmic rays. The name, cosmic rays was given by Millikan.47.What are primary cosmic rays?The primary cosmic rays are those comingfrom outer space and enter the outer boundary of the earth’satmosphere. The primary cosmic rays consist of 90% protons, 9%helium nuclei and remaining heavy nuclei. The energy of the primarycosmic rays is of the order 108 MeV.48.What are secondary cosmic rays?The secondary cosmic rays are produced when primary cosmicrays interact with gases in the upper layers of the atmosphere. Theyare made up of particles like α-particles, protons, electrons, positrons,mesons, photons, etc. in different proportions.49.What is Latitude effect of cosmic rays?The variation of cosmic ray intensity with geomagnetic latitude is known as latitude effect. The experiments to study the variation of cosmic ray intensity (I) with geomagnetic latitude (θ) showed that the intensity is maximum at the poles (θ = 900), minimum at the equator (θ = 0) and constant between latitudes of 420 and 90050.What is altitude effect of cosmic rays?The study of variation of cosmic ray intensity (I) with altitude (h) isknown as altitude effect. It is seen that the intensity increases with altitude and reaches a maximum at a height of about 20 km.51..What is Pair production and annihilation of matter?The conversion of a photon into an electron−positron pair on itsinteraction with the strong electric field surrounding a nucleus iscalled pair production.The converse of pair production in which an electron and positroncombine to produce a photon is known as annihilation of matter.